Control embedded machine condition monitor

ABSTRACT

Analog signals are generated in response to one or more operating parameters of a machine such as vibration and used to provide a log of the operating history of the machine. The analog signals are converted into root mean square (rms) values which are periodically sampled. The sampled signals are sorted into predefined rms value bands. Each occurrence of a signal in an rms value band is used to increment a counter in order to keep track of the number of occurrences of signals in a particular rms value band. The number of accumulated signals in each of the rms value bands provides an indication of the usage and condition of the machine.

This application claims the benefit of Provisional Application No.60/250,621, filed Dec. 1, 2000.

BACKGROUND OF THE INVENTION

The present invention relates in general to methods and apparatus formonitoring the operation and use of a machine. More particularly, thepresent invention relates to methods and apparatus for monitoring theoperation and use of a machine for the purpose of diagnosing themechanical condition of the machine based thereon.

It is known to monitor machine operation parameters, such as, forexample, machine vibration, temperature, sound, force or pressure, motorcurrent or lubricant oil temperature, viscosity or composition, and toevaluate such monitored parameters for the purpose of providing anindication of the condition of the machine. For example, a thermocouplemay be located within the lubricant “bath” so as to alert the machineoperator when the temperature of the oil has exceed some predeterminedthreshold value. In another example relating to machine vibration,techniques employed to monitor and to analyze machine vibration for thepurpose of providing an indication of the condition of the machine rangefrom the very simple, such as touching or listening to a machine, to thevery complicated, such as detecting time-domain signals, convertingthose signals to frequency-domain signals, and computer-processing theresulting frequency domain signals. Techniques at the lower range of themonitoring and analysis spectrum are overly-simplified and do notprovide reliable results whereas techniques at the upper range of thespectrum are overly-complicated and are typically so expensive so as toprohibit their widespread use by most machine operators.

Accordingly, there is a need for new and improved monitoring andanalysis techniques for providing an indication of the condition of amachine, such as an indication of the general “health” of the machine byproviding a running history or profile of the machine's operation orusage. Preferably, such monitoring and analysis techniques would providereliable results, would be easy to use, would be relatively inexpensiveand would be adaptable to improve the operating practices of themachine, such as, for example, as one component of the machineoperator's predictive/preventative maintenance procedures. There also isa need for new and improved analysis techniques for providing anindication of the useful “life” remaining in a machine, based uponactual past machine operation and usage.

SUMMARY OF THE INVENTION

These and other needs are met by the present invention wherein analogsignals generated in response to one or more operating parameters of amachine, such as, for example, vibration, temperature, sound, force,pressure, motor current or lubricant oil temperature, viscosity orcomposition, are converted into root-mean-square (“rms”) values whichare periodically sampled, sorted into predefined rms value bands andstored as an evolving history log of the actual operation of themachine. Data stored in a machine operation history log may be analyzedto evaluate past machine operation and usage, to provide an indicationof the general “health” of the machine, or to provide an indication ofthe useful “life” remaining in the machine. In addition, data stored inthe log may be analyzed as a component of a machine operator'spredictive/preventative maintenance procedure.

In accordance with one aspect of the present invention, a machine ismonitored by coupling a transducer to the machine at a location thereonlikely to detect changes in the level of one or more operatingparameters of the machine's operation. The transducer preferably isadapted to generate analog signals representing the level of theoperating parameter detected. The analog signals are converted to rmssignals which are periodically sampled to generate rms sample values.The rms sample values are processed according to each sample value'srespective level or magnitude and thereafter sorted into two or moresignal magnitude bands, where the number of rms samples detected in eachof the at least two signal magnitude bands is accumulated over time toprovide an indication of the usage and condition of the machine.

In accordance with another aspect of the present invention, a machine ismonitored by coupling a transducer to the machine at a location thereonlikely to detect changes in the level of one or more operatingparameters of the machine's operation. The transducer preferably isadapted to generate analog signals representing the level of theoperating parameter detected and is operatively coupled to a machinecontroller of the machine. The analog signals are converted to rmssignals, which the controller periodically samples and generates rmssample values corresponding thereto. The controller then processes therms sample values according to each sample value's respective level ormagnitude, sorts the rms sample values into sample magnitude bands,accumulates the number of rms sample values into each of at least twomagnitude bands and uses the accumulated numbers of accumulated rmssamples in the at least two magnitude bands to provide an indication ofthe usage and condition of the machine.

In accordance with yet another aspect of the present invention, amachine is monitored by monitoring analog signals representative ofoperation of the machine. The analog signals are converted to rmssignals that are periodically sampled to generate rms sample values. Therms sample values are processed according to each rms sample value'smagnitude or level and sorted into two or more sample magnitude bands.The numbers of rms sample values are accumulated in each of at least twosample magnitude bands and the numbers of accumulated rms sample valuesin the at least two sample magnitude bands are used to provide anindication of the usage and condition of the machine.

In accordance with still another aspect of the present invention, anapparatus for monitoring a machine having a controller comprises atransducer coupled to the machine. Circuitry is provided for convertinganalog signals received from the transducer to rms signals. The machinecontroller is then operated to perform the functions of: periodicallysampling the rms signals to generate rms sample values; sorting the rmssample values according to each rms sample value's level or magnitude;accumulating the number of rms sample values in each of at least twomagnitude bands; and, providing an indication of the usage and conditionof the machine from the accumulated numbers of rms sample valuesaccumulated in at least one of the at least two magnitude bands.

These and additional objects, features and advantages of the presentinvention will become apparent to those reasonably skilled in the artfrom the description which follows, and may be realized by means of theinstrumentalities and combinations particularly pointed out therein, aswell as by those instrumentalities and combinations equivalent thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be had uponreference to the following description in conjunction with theaccompanying drawings in which like numerals refer to like parts, andwherein:

FIG. 1 is a schematic side view of an exemplary machine tool having anapparatus according to a preferred embodiment of the present inventioninstalled thereon;

FIG. 2 is a flow chart showing operation of a preferred method ofpracticing the present invention;

FIG. 3 is an exemplary illustration of a display showing accumulatedmachine tool spindle vibration acceleration collected and displayed inaccordance with a preferred method of practicing the present invention;and,

FIG. 4 is an exemplary illustration of a display showing real-timemachine tool spindle vibration acceleration and peak-value maximumvibration acceleration monitored, collected and displayed in accordancewith a preferred method of practicing the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be describedwith reference to the various drawing figures, wherein FIG. 1 is aschematic side view of an exemplary machine tool, such as, for example,a vertical machining center 100. It will be apparent to those ofordinary skill in the art, upon reading the within description, that theapparatus and method according to the various embodiments of the presentinvention may be practiced with respect to machines of alternativeconfigurations, structures and operations without departing from eitherthe spirit or the scope of the present invention. For example, theapparatus and method according to the various embodiments of the presentinvention may be used on any combination of other machine tools, such ashorizontal machining centers, turning centers, milling machines,profilers, routers, planers, boring mills, drills, broaches,gear-cutters, screw-cutters, shapers, grinders, press brakes or reamers,on any combination of composite-material forming machinery such asfiber-placement machines or composite tape-laying machines, on anycombination of robotic machinery or on any combination of any other typeof machinery where there is a need or desire to obtain an indication ofthe usage and condition thereof.

According to a preferred embodiment of the present invention, one ormore signal transducers (which may be analog or digital, but preferablyanalog) are used to monitor any combination of one or more operatingparameters or characteristics of a machine's operation, such as, forexample, vibration, temperature, sound, force, pressure, motor currentor lubricant oil temperature, viscosity or composition. Output signalsfrom the transducers are processed to provide an inexpensive view of therunning history of the usage and operation of the machine 100. For easeof illustration, a preferred embodiment of the present invention will bedescribed with reference to a system wherein a single transducer is usedand is selected to be an accelerometer 102 mounted to a machine toolspindle 104 for the purpose of monitoring acceleration forces exerted onthe spindle during machining operations. Additional transducers may becoupled to other portions of the machine to monitor othercharacteristics of machine operation as required for a givenapplication, as will be apparent to those of ordinary skill in the art.In addition to the history of machine operation, real-time machineoperating conditions may be monitored, viewed and analyzed to provide anoperator with an indication of the real-time operating conditions of themachine and to alert the operator of any significant operating events,such as excessive vibration acceleration, lubricant oil temperature orthe like.

FIG. 1 shows an apparatus coupled to the machine 100 for use accordingto a preferred embodiment hereof wherein the accelerometer 102 ispowered from a power source located inside an electrical cabinet 106coupled to the machine 100 in a conventional manner. Analog signalsgenerated by the accelerometer 102 are conditioned by a first electricalcircuit 108 and passed to voltage measuring and analog-to-digitalconverter (“ADC”) circuitry 110 so that digital signals representativeof the acceleration forces are passed to a computerized numericcontroller 112 for further processing. The first electrical circuit 108includes amplification and root-mean-square (“rms”) signal generatingcomponents. It is noted that circuitry performing the functions of thefirst electrical circuitry 108 can be included in the accelerometer 102so that rms signals are generated directly by the accelerometer 102rather than by a separate circuit such as first circuit 108.Accelerometers of both types are commercially available from a number ofmanufacturers including, for example, PCB Peizotronics of Depew, N.Y.Normal control sampling of the rms signals at a relatively low samplerate is adequate to represent, over time, the operating conditions ofthe machine 100. For this reason, ADC circuitry 110 may take the form ofan inexpensive, commercially-available computer interface card since ahigh sample rate digital signal processing board (as is commonlyemployed for sophisticated vibration signal analysis in existingsystems) is not required. For example, the present inventioncontemplates adequate sampling rates ranging from one sample everymillisecond to one sample every 10 seconds. Alternatively, rather thanproviding separate ADC circuitry 110, rms sample values may be suppliedby first circuit 108 directly to the machine controller 112 andprocessed in accordance with the present invention. Even further, ratherthan providing separate first circuit 108, rms sample values may besupplied by accelerometer 102 directly to the machine controller 112 andprocessed in accordance with the present invention.

In any event, rms sample values representative of acceleration forcesexerted on the spindle 104 are received by the controller 112 andprocessed to quantify the general severity of operation of the machine100. Since rms signal values are used, short duration events, such asimpulses, may not be detected, due to the low sampling rate. Similarly,peak amplitudes of some events may go undetected. However, the generaltrend of the characteristics and conditions of machine operation and usecan be ascertained from the rms sample values acquired at somepredetermined sample rate over time.

Currently available analysis systems known to applicants requiresophisticated computer processors equipped with a great amount ofcomputer memory to store data for multiple vibration spectra. If onlylimited memory is available to acquire, store and analyze sample data,only the overall levels or frequency band levels can be monitored totrend the data over time. As overall levels increase, an alarm isprovided to indicate a deteriorating condition of the machine. Whilesuch monitoring is adequate to detect wear- or fatigue-induced equipmentfailures of machinery having relatively stable and consistent operatingconditions, it generally is not adequate for machines that are operatedunder non-stable or varying conditions. That is, traditionalfatigue-analysis procedures presuppose a substantially continuousoperating condition with operating characteristics that do not varysignificantly over time. On the other hand, typical machine tool usagesubjects the machine 100 to widely-varying operating conditions. Forexample, utilization of different tooling and/or different depths of cutmay place different levels of vibration acceleration on the machine 100to create greater or lesser degrees of use or abuse thereof. To thisend, the present invention is suitable to monitor machine operations andoperating severity for stable and consistent operating conditions, aswell as for non-stationary or varying operating conditions. An advantageof the present invention is that it does not require the use of astandard “maintenance cycle” to assess the condition of the machine, butrather may assess operating conditions based on accumulated datarepresenting the actual use and operation of the machine 100.

An exemplary method according to a preferred embodiment of the presentinvention may be implemented on a computer or computer processor havingonly a limited amount of computer memory, although the methods andapparatus described herein can be implemented on more powerful computersas well. As will be discussed is greater detail below, the computer needonly be suitable for storing single numbers for each of two or moreacceleration force bands and/or for incrementing two or more counters toaccumulate data within selected bands of vibration acceleration.Moreover, by using the sampling rate of the controller 112 such as inone alternative embodiment hereof, time periods of exposure of themachine 100 to measure acceleration levels of the spindle 104 can alsobe estimated.

With reference now to FIG. 2, a method of practicing the presentinvention according to a preferred embodiment thereof will be describedin further detail. As described above, the transducer is in the form ofan accelerometer 102 coupled to a machine tool spindle 104 and isadapted to output an analog signal 103 representing the level ormagnitude of acceleration forces exerted on the spindle 104. Firstcircuit 108 continuously receives the analog signal 103 and conditionsthe analog signal 103, for example, by amplifying it. Circuit 108 alsoperiodically calculates time-averaged root-mean-square values 109 of thesignal 103 and stores the rms value 109 until such time as a new rmsvalue 109 is calculated, at which time, the new rms value 109 replacesthe old rms value.

At some predetermined sample rate, for example, one sample per second,programmed into ADC circuit 110, the ADC circuit 110 obtains, acquiresor otherwise samples the current value of the rms value 109 of theanalog signal 103 and determines the level or magnitude of the rms value109. The ADC circuit 110 then supplies the machine controller 112 withthe sampled rms value 109, and more preferably, with the absolute valueof the sampled rms values 109.

The machine controller 112 is programmed to include two or morebandwidth counter “bins”, which store counter values corresponding topreselected bands—or ranges—of sampled rms values 109 and to sort themagnitudes of the sampled rms values 109 into the two or more bandwidthcounters. For example, machine controller 112 provides a first bandwidth“bin” for counting the number of rms sample values 109 falling within afirst range between 0 g and 15 g, a second bandwidth “bin” for countingthe number of rms sample values 109 falling within a second rangebetween 15 g and 30 g, a third bandwidth “bin” for counting the numberof rms sample values 109 falling within a third range between 30 g and40 g and a fourth bandwidth “bin” for counting the number of rms samplevalues 109 greater than 40 g. As the machine controller 112 receiveseach rms sample value 109, the machine controller 112 compares the rmssample value 109 to the predetermined bandwidth “bin” ranges. Dependingon which bandwidth “bin” the rms sample value 109, falls in, the machinecontroller 112 increments the corresponding bandwidth “bin” counteraccordingly.

For example, if the machine controller 112 determines that the rmssample value 109 is between 0 g and 15 g, a first bandwidth counter CTR1is incremented by a value of one. If, instead, the machine controller112 determines that the rms sample value 109 is between 15 g and 30 g, asecond bandwidth counter CTR2 is incremented by a value of one.Similarly, if the machine controller 112 determines that the rms samplevalue is between 30 g and 40 g, or is greater than 40 g, a third orfourth counter CTR3, CTR4, respectively, is incremented by a value ofone. As each counter is incremented according to the rms sample values109, the ADC circuit 110 obtains additional rms sample values 109. Inthis manner, the counter values accumulated in each of the bandwidth“bins” represent measures of the trends of the machine 100 to operateunder certain operating conditions. Any of the counters may be reset tozero, for example, where a new spindle 104 is installed on the machine100 and where retention of operational history data acquired theretoforeconcerning the prior spindle is not required.

While the preferred embodiment hereof has been described with referenceto four bandwidth “bins”, any number of “bins” representing greater orlesser ranges may be utilized as the machine operator deems fit for theparticular type and general operating environment of the machine. Oneadvantage of the present invention is that substantial computerresources, such as memory, are not required in order to practice themethod hereof. For example, because the computer is storing only thecounter values, a single memory register for each of the bandwidth“bins” is all that is required. Vast computer memory resources (such asare used in the prior art to store vibration spectra) are not necessary.

Referring now to FIG. 3, data accumulated for each of these bands 120may be displayed on a display 114 of the controller 112 where cumulativedata is displayed in the form of a first histogram 114 a. In addition,referring also to FIG. 4, a second histogram 114 b may be displayed onthe display 114 to show the magnitude of the last-sampled rms level 130,the maximum rms level 132 sampled during some predetermined period, aswell as other desired data (not shown) relating to machine operation. Toprovide the machine operator with useful information concerning thenature of operating conditions exceeding some predetermined thresholdvalue, a limited number of event type descriptions can be stored withthe data. Similarly, since the system is embedded on the machine, i.e.,incorporated into the controller 112 rather than provided in a separatecomputer control, alarms and alerts can be generated by the controller112 based upon data sampled from the transducer to notify the machineoperator of critical operating events, such as a spindle vibrationacceleration exceeding some predetermined threshold value.

The data accumulated in the present invention can also be used tocalculate machine “expended life” or “life remaining” measure similar tothe known “accumulated fatigue damage factor (“AFDF”) and/or “L₁₀” or“B₁₀” bearing life calculations. Typical “expended life” or “liferemaining” calculations are based on cyclical stress, so the number ofcycles at a particular stress level is required. In the presentinvention, the number of stress cycles is not established exactly, sincedata is not sampled continuously and as such, the number of cycles isnot recorded precisely. However, a number of methods may be used toestimate accumulated damage based upon data sampled according to thepresent invention.

One method presumes that the cycles are predominantly due to suchfactors as: the rotational speed of the spindle 104, the number ofimpacts of the cutting edge of the tool on the workpiece, and/or thenatural frequency of the machine 100. Because the apparatus is embeddedwithin the controller 112, other measurements can be associated with themeasured spindle vibration acceleration, such as spindle speed, horsepower, feedrate of axes, axis motor current and the like and used aloneor in combination with one another to generate a stress profile. Theselection of a particular tool used and its “force profile” can also beassociated with vibration measurements.

Machine evaluation techniques based on the baseline empiricalmeasurements can be utilized in a “learn cycle” for use in a damageassessment algorithm. Fuzzy logic type algorithms would be appropriatefor such machine evaluation techniques. For example see: $\begin{matrix}{{{Accumulated}\quad{Fatigue}\quad{Damage}\quad{{Factor}( {``{AFDF}"} )}} = {\sum\limits_{j = 1}^{\sigma = x}\quad( {{n_{1}\sigma_{1}^{b}} + \ldots + {n_{x}\sigma_{x}^{b}}} )}} & ( {{Eq}.\quad 1} )\end{matrix}$

-   -   where: n=number of stress events;    -    σ=magnitude of the stress event; and,    -    b=slope of log-log S-N curve

For the present invention, the use of information readily availablewithin the controller 112 is expanded. That is, the number of stressevents, n, will be defined to include stress events estimated fromseveral measures, such as spindle speed, number of teeth on cutter,empirically-determined predominant vibration, and frequency. The numberof stress events, n, then, can be used in Eq. 1 to calculate the AFDF,which may then used in a conventional manner to determined theaccumulated fatigue damage suffered by the machine.

For example:

-   -   n (rpm, m)=number at assumed cycles of the acceleration margin        predominant vibration assumed to be tooth impacts so, if spindle        speed=2000 rpm and number of cutter teeth m=6 then:        -   n=2000 rpm×6 teeth=12,000 cycles/min

Thus, for every minute at 10 g's rms operating at 2000 rpm with thiscutter, 12,000 stress cycles of 10 g magnitude are presumed.

Similarly, the stress events can be defined by operation severitycalculations, based not only on the vibration acceleration level, butalso the combined dosage of spindle horsepower, axis thrust, vibration,etc.

σ(x, hp, I_(axis))

Such an embedded algorithm is then be used to indicate remaining life:

Expected life−AFDF use=remaining life.

While the invention has been illustrated with reference to one or morepreferred embodiments hereof, and such preferred embodiments have beendescribed in considerable detail with reference to the drawings, it isnot the intention of applicants that the invention be restricted to suchdetail. Rather, it is the intention of the applicants that the inventionbe defined by all equivalents of the preferred embodiments fallingwithin the scope hereof.

1. A method for evolving a history log of the operating history of amachine that is subjected to non-stationary or varying operatingconditions characterized in that said method comprises the steps of:coupling a transducer to the machine, said transducer generating signalsrepresentative of the operation of said machine; converting said signalsto a root mean square (rms) signal; periodically sampling said rmssignal to generate rms samples and processing said rms samples accordingto rms sample magnitude; sorting said rms samples into signal magnitudebands; incrementing a bandwidth counter by a value of one for each rmssample contained within a signal magnitude band; and, using the countervalue of each of the bandwidth counters to create an output indicationof the operational history of the machine.
 2. A method for monitoringthe operational history of a machine using a machine controller,characterized in that said method comprises the steps of: coupling atransducer to a machine to be monitored, said transducer generatingsignals representative of the operation of said machine; converting saidsignals to a root mean square (rms) signal; coupling said rms signal toa controller of said machine; operating said controller to periodicallysample said rms signal and generate rms samples; operating saidcontroller to process said rms samples according to sample magnitude;operating said controller to sort said rms samples into signal magnitudebands; incrementing a bandwidth counter by a value of one for each rmssample contained within a signal magnitude band; and, using the countervalue of each of the bandwidth counters to develop a display indicativeof the usage and condition of said machine.
 3. A method for monitoringthe operational history of a machine, characterized in that said methodcomprises the steps of: monitoring signals representative of theoperation of said machine and converting said signals to a root meansquare (rms) signal; periodically sampling said rms signal to generaterms samples; processing said rms samples according to sample magnitude;sorting said rms samples into signal magnitude bands; accumulating thenumber of rms samples in each of at least two magnitude bands;incrementing a bandwidth counter by a value of one for each rms signalcontained within a signal magnitude band; and, using the bandwidthcounter value to determine the usage and condition of said machine. 4.An apparatus for monitoring the operational history of a machine that issubjected to non-stationary or varying operating conditions having acontroller, said apparatus comprising: a transducer coupled to saidmachine; and, circuitry for converting signals supplied to saidcircuitry from said transducer to root mean square (rms) signals;characterized in that said controller of said machine is operated toperform the functions of: sampling said rms signals to generate rmssamples; sorting said rms samples according to sample magnitude;accumulating in a bandwidth counter the number of rms samples in each ofat least two magnitude bands; and, determining the usage and conditionof said machine from the accumulated number in the bandwidth counter forat least one of said at least two magnitude bands.
 5. The method formonitoring a machine as claimed in claim 1 wherein said step ofperiodically sampling said rms signal comprises the step of samplingsaid rms signal at a known sampling rate.
 6. The method for monitoring amachine as claimed in claim 1, wherein said machine comprises a machinetool and said step of coupling a transducer to said machine toolcomprises the step of coupling an accelerometer to a spindle of saidmachine tool to monitor vibration accelerations of said spindle.
 7. Themethod for monitoring a machine as claimed in claim 1 further comprisingthe step of displaying the accumulated numbers of rms samples in atleast one of said at least two magnitude bands.
 8. The method formonitoring a machine as claimed in claim 1 wherein said step of saidtransducer generating said signals further comprises the step ofgenerating analog signals.
 9. The method for monitoring a machine asclaimed in claim 1 wherein said step of said transducer generating saidsignals further comprises the step of generating digital signals. 10.The method for monitoring a machine as claimed in claim 2 furthercomprising the step of operating said controller to display theaccumulated numbers of rms samples in said at least two magnitude bands.11. The method for monitoring a machine as claimed in claim 2 whereinsaid step of said transducer generating said signals further comprisesthe step of generating analog signals.
 12. The method for monitoring amachine as claimed in claim 2 wherein said step of said transducergenerating said signals further comprises the step of generating digitalsignals.
 13. The method for monitoring a machine as claimed in claim 3wherein said step of monitoring signals further comprises the step ofmonitoring analog signals.
 14. The method for monitoring a machine asclaimed in claim 3 wherein said step of said monitoring signals furthercomprises the step of monitoring digital signals.
 15. The apparatus formonitoring a machine as claimed in claim 4 wherein said controllerfurther comprises a display for displaying the accumulated number of rmssamples in at least one of said at least two magnitude bands.
 16. Theapparatus for monitoring a machine as claimed in claim 4, wherein saidsignals supplied to said circuitry from said transducer are analogsignals.
 17. The apparatus for monitoring a machine as claimed in claim4, wherein said signals supplied to said circuitry from said transducerare digital signals.